Project summary. Zinc is key to many of biological processes including gene expression and enzymatic activity, however, it is toxic at high levels. Exposure of cells to high levels of zinc leads to the loss of enzyme activity, generation of reactive oxygen species (ROS), and activation of apoptosis. Excess zinc is especially toxic to the mitochondria, due to its ability to inhibit several components of electron transfer chain. In many organelles zinc concentration is regulated by a system of transposers; however, such as system has not been shown for the mitochondria. Using a combination of bioinformatics and cell biological assays we have identified ZnT9 (SLC30A9) as a candidate mitochondrial transporter. The overarching goal of this proposal is to test the hypothesis that ZnT9 is a critical regulator of mitochondrial zinc under control and pathological conditions, and that its dysfunction can lead to neuronal injury. We propose that under normal conditions, the mitochondrial proton gradient powers zinc expulsion from the mitochondria, limiting zinc toxicity and providing a neuroprotection function. We think that in damages mitochondria ZnT9 is reversed, accelerating the damage. This is a novel function for a zinc transporter and a new mechanism of neuroprotection as well as neurotoxicity. To test this model, we will pursue two specific aims. Aim 1 of the present project is focused on identifying the role of ZnT9 in mitochondrial zinc fluxes under normal conditions and in damaged mitochondria. Aim 2 will answer whether ZnT9 is cytoprotective under zinc overload and oxidative stress conditions, and whether ZnT9 reversal accelerates neuronal cells death. Our proposed studies will likely establish a new molecular determinant of mitochondrial zinc transport and a previously unrecognized component of zinc neurotoxicity. The completion of the studies proposed in this Exploratory/Development Research Program will likely provide the rationale for future investigations of ZnT9 function and dysfunction in in vivo mouse models and an exploration of the possible link of this transporter to human neurodegenerative disorders.